Looks like Mark Chu-Carroll has finally seen the light

Mark added this to his post about the downwind-faster-than-wind problem today:

It appears that I really blew it with this one. I’m the bozo in this story. After lots of discussion, a few equations, and a bunch of time scribbling on paper, I’m convinced that I got this one wrong in a big way. No excuses; I should have done the analysis much more carefully before posting this; looking back, what I did do was pathetically shallow and, frankly, stupid. I’m sincerely sorry for calling the guys doing the experiment bozos. I’ll follow up later this weekend with a detailed post showing my analysis, where I screwed up, and why this thing really works. In the meantime, feel free to call me an idiot in the comments; I pretty much deserve it. I’m leaving the post here, with this note, as a testament to my own stupidity and hubris in screwing this up.

Bravo, Mark, for doing the right thing here. I’m looking forward to seeing the analysis.

3 Responses to Looks like Mark Chu-Carroll has finally seen the light

I’m glad too. The tacking and “ever larger sails keep adding energy to the system” arguments convinced me that it was in principle possible, and I didn’t think the cart-builders were hoaxers, but I was not able to do the math right.

Mark has posted his analysis. See “The Real Bozo Attempts to Atone: Why the DDWFTW Car Works” on his blog.

There’s a problem, though: his analysis isn’t correct. I think he’s jumped in too fast again. It’s great that he admits his first mistake so openly and honestly, but maybe he should have taken a little more time before showing everybody “how it really works”.

First he analyses a mechanical cart and comes to the conclusion that such a cart can’t work. It’s clear that his particular cart won’t work as hoped: it will run, but at half the speed of the belt that pushes it (as several people have pointed out in the comments). There are indeed mechanical models that work fine, not only in theory but also in practice. See my minimalist version at http://www.youtube.com/watch?v=E7vcQcIaWSQ

He concludes that, since the mechanical versions “don’t work”, the wind version is different: it “isn’t rigid”, so it behaves differently. In fact the mechanical models and the DDWFTTW cart use exactly the same principle: you need to push/pull (with a conveyor belt, a strip of moving paper, a stream of moving air…) at part of the cart that is moving forward slower than the cart as a whole. The fun is working out how to get part of the cart to move forward slower than the rest of it.